It is known to control the vertical vibration of the sprung member of a vehicle by interposing an actuator between the sprung member and the unsprung member, and causing a relative movement between the two members by producing a drive force with the actuator according to the skyhook theory. See JP2001-311452A, for instance.
However, when the vertical vibration of the sprung member is extensively controlled by controlling such an actuator, the vibrations which are caused by rumble strips and bumps formed on the surface for the purpose of eliciting the attention of the vehicle operator may be controlled to such an extent that the vehicle operator's attention may not be elicited as intended.